A challenge for semiconductor processing engineers is the formation of submicron contacts and vias with high aspect ratios. Forming the contacts and vias with aluminum is preferred over tungsten because of aluminum's lower resistance, fewer overall process steps, and improved electromigration performance. However, aluminum reflow for contact or via filling has not been widely accepted due to the higher deposition temperature and difficulty in completely filling the high aspect ratio contacts and vias. With the advent of a force-fill.TM. process developed by Electrotech Ltd. of Bristol, United Kingdom, a metal, typically aluminum, sputtered onto the semiconductor surface can be forcibly extruded into the small via or contact openings by exerting high pressure at an elevated temperature. Although this force-fill.TM. process enhances the filling of the via and contact recesses and openings, the force-fill process also creates substantial cracks or splits in the metal film surface.
The fill metal surface splitting or cracking is similar in appearance to the cracks in dry parched earth. The cracks may extend substantially into the metal film as to interfere with the conduction of current therethrough. Further, particulate defects and process induced defects on patterned silicon wafers are generally detectable with the use of optical defect detection equipment and laser scatter defect detection equipment. However, the detection equipment is more likely to generate erroneous detection results due to the grain boundary cracks. These cracks trap a large percentage of the incident light or laser, which causes them to appear as dark and ragged-edged lines. These dark lines are detected by the optical and laser scatter equipment as nuisance defects, making detection of true particulate defects difficult without significantly reducing the effective sensitivity of the equipment.